309 lines
12 KiB
C++
309 lines
12 KiB
C++
//===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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///
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/// \file
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/// Fix bitcasted functions.
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///
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/// WebAssembly requires caller and callee signatures to match, however in LLVM,
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/// some amount of slop is vaguely permitted. Detect mismatch by looking for
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/// bitcasts of functions and rewrite them to use wrapper functions instead.
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///
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/// This doesn't catch all cases, such as when a function's address is taken in
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/// one place and casted in another, but it works for many common cases.
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///
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/// Note that LLVM already optimizes away function bitcasts in common cases by
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/// dropping arguments as needed, so this pass only ends up getting used in less
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/// common cases.
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///
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//===----------------------------------------------------------------------===//
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#include "WebAssembly.h"
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#include "llvm/IR/CallSite.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Module.h"
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#include "llvm/IR/Operator.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace llvm;
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#define DEBUG_TYPE "wasm-fix-function-bitcasts"
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static cl::opt<bool>
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TemporaryWorkarounds("wasm-temporary-workarounds",
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cl::desc("Apply certain temporary workarounds"),
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cl::init(true), cl::Hidden);
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namespace {
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class FixFunctionBitcasts final : public ModulePass {
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StringRef getPassName() const override {
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return "WebAssembly Fix Function Bitcasts";
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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ModulePass::getAnalysisUsage(AU);
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}
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bool runOnModule(Module &M) override;
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public:
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static char ID;
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FixFunctionBitcasts() : ModulePass(ID) {}
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};
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} // End anonymous namespace
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char FixFunctionBitcasts::ID = 0;
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INITIALIZE_PASS(FixFunctionBitcasts, DEBUG_TYPE,
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"Fix mismatching bitcasts for WebAssembly", false, false)
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ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() {
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return new FixFunctionBitcasts();
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}
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// Recursively descend the def-use lists from V to find non-bitcast users of
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// bitcasts of V.
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static void FindUses(Value *V, Function &F,
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SmallVectorImpl<std::pair<Use *, Function *>> &Uses,
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SmallPtrSetImpl<Constant *> &ConstantBCs) {
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for (Use &U : V->uses()) {
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if (BitCastOperator *BC = dyn_cast<BitCastOperator>(U.getUser()))
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FindUses(BC, F, Uses, ConstantBCs);
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else if (U.get()->getType() != F.getType()) {
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CallSite CS(U.getUser());
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if (!CS)
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// Skip uses that aren't immediately called
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continue;
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Value *Callee = CS.getCalledValue();
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if (Callee != V)
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// Skip calls where the function isn't the callee
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continue;
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if (isa<Constant>(U.get())) {
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// Only add constant bitcasts to the list once; they get RAUW'd
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auto c = ConstantBCs.insert(cast<Constant>(U.get()));
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if (!c.second)
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continue;
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}
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Uses.push_back(std::make_pair(&U, &F));
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}
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}
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}
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// Create a wrapper function with type Ty that calls F (which may have a
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// different type). Attempt to support common bitcasted function idioms:
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// - Call with more arguments than needed: arguments are dropped
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// - Call with fewer arguments than needed: arguments are filled in with undef
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// - Return value is not needed: drop it
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// - Return value needed but not present: supply an undef
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//
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// If the all the argument types of trivially castable to one another (i.e.
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// I32 vs pointer type) then we don't create a wrapper at all (return nullptr
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// instead).
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//
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// If there is a type mismatch that we know would result in an invalid wasm
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// module then generate wrapper that contains unreachable (i.e. abort at
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// runtime). Such programs are deep into undefined behaviour territory,
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// but we choose to fail at runtime rather than generate and invalid module
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// or fail at compiler time. The reason we delay the error is that we want
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// to support the CMake which expects to be able to compile and link programs
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// that refer to functions with entirely incorrect signatures (this is how
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// CMake detects the existence of a function in a toolchain).
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//
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// For bitcasts that involve struct types we don't know at this stage if they
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// would be equivalent at the wasm level and so we can't know if we need to
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// generate a wrapper.
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static Function *CreateWrapper(Function *F, FunctionType *Ty) {
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Module *M = F->getParent();
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Function *Wrapper = Function::Create(Ty, Function::PrivateLinkage,
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F->getName() + "_bitcast", M);
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BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
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const DataLayout &DL = BB->getModule()->getDataLayout();
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// Determine what arguments to pass.
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SmallVector<Value *, 4> Args;
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Function::arg_iterator AI = Wrapper->arg_begin();
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Function::arg_iterator AE = Wrapper->arg_end();
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FunctionType::param_iterator PI = F->getFunctionType()->param_begin();
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FunctionType::param_iterator PE = F->getFunctionType()->param_end();
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bool TypeMismatch = false;
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bool WrapperNeeded = false;
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Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
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Type *RtnType = Ty->getReturnType();
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if ((F->getFunctionType()->getNumParams() != Ty->getNumParams()) ||
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(F->getFunctionType()->isVarArg() != Ty->isVarArg()) ||
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(ExpectedRtnType != RtnType))
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WrapperNeeded = true;
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for (; AI != AE && PI != PE; ++AI, ++PI) {
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Type *ArgType = AI->getType();
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Type *ParamType = *PI;
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if (ArgType == ParamType) {
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Args.push_back(&*AI);
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} else {
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if (CastInst::isBitOrNoopPointerCastable(ArgType, ParamType, DL)) {
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Instruction *PtrCast =
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CastInst::CreateBitOrPointerCast(AI, ParamType, "cast");
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BB->getInstList().push_back(PtrCast);
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Args.push_back(PtrCast);
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} else if (ArgType->isStructTy() || ParamType->isStructTy()) {
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LLVM_DEBUG(dbgs() << "CreateWrapper: struct param type in bitcast: "
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<< F->getName() << "\n");
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WrapperNeeded = false;
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} else {
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LLVM_DEBUG(dbgs() << "CreateWrapper: arg type mismatch calling: "
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<< F->getName() << "\n");
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LLVM_DEBUG(dbgs() << "Arg[" << Args.size() << "] Expected: "
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<< *ParamType << " Got: " << *ArgType << "\n");
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TypeMismatch = true;
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break;
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}
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}
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}
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if (WrapperNeeded && !TypeMismatch) {
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for (; PI != PE; ++PI)
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Args.push_back(UndefValue::get(*PI));
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if (F->isVarArg())
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for (; AI != AE; ++AI)
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Args.push_back(&*AI);
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CallInst *Call = CallInst::Create(F, Args, "", BB);
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Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
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Type *RtnType = Ty->getReturnType();
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// Determine what value to return.
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if (RtnType->isVoidTy()) {
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ReturnInst::Create(M->getContext(), BB);
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} else if (ExpectedRtnType->isVoidTy()) {
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LLVM_DEBUG(dbgs() << "Creating dummy return: " << *RtnType << "\n");
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ReturnInst::Create(M->getContext(), UndefValue::get(RtnType), BB);
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} else if (RtnType == ExpectedRtnType) {
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ReturnInst::Create(M->getContext(), Call, BB);
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} else if (CastInst::isBitOrNoopPointerCastable(ExpectedRtnType, RtnType,
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DL)) {
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Instruction *Cast =
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CastInst::CreateBitOrPointerCast(Call, RtnType, "cast");
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BB->getInstList().push_back(Cast);
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ReturnInst::Create(M->getContext(), Cast, BB);
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} else if (RtnType->isStructTy() || ExpectedRtnType->isStructTy()) {
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LLVM_DEBUG(dbgs() << "CreateWrapper: struct return type in bitcast: "
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<< F->getName() << "\n");
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WrapperNeeded = false;
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} else {
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LLVM_DEBUG(dbgs() << "CreateWrapper: return type mismatch calling: "
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<< F->getName() << "\n");
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LLVM_DEBUG(dbgs() << "Expected: " << *ExpectedRtnType
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<< " Got: " << *RtnType << "\n");
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TypeMismatch = true;
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}
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}
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if (TypeMismatch) {
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// Create a new wrapper that simply contains `unreachable`.
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Wrapper->eraseFromParent();
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Wrapper = Function::Create(Ty, Function::PrivateLinkage,
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F->getName() + "_bitcast_invalid", M);
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BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
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new UnreachableInst(M->getContext(), BB);
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Wrapper->setName(F->getName() + "_bitcast_invalid");
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} else if (!WrapperNeeded) {
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LLVM_DEBUG(dbgs() << "CreateWrapper: no wrapper needed: " << F->getName()
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<< "\n");
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Wrapper->eraseFromParent();
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return nullptr;
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}
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LLVM_DEBUG(dbgs() << "CreateWrapper: " << F->getName() << "\n");
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return Wrapper;
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}
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bool FixFunctionBitcasts::runOnModule(Module &M) {
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Function *Main = nullptr;
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CallInst *CallMain = nullptr;
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SmallVector<std::pair<Use *, Function *>, 0> Uses;
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SmallPtrSet<Constant *, 2> ConstantBCs;
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// Collect all the places that need wrappers.
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for (Function &F : M) {
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FindUses(&F, F, Uses, ConstantBCs);
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// If we have a "main" function, and its type isn't
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// "int main(int argc, char *argv[])", create an artificial call with it
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// bitcasted to that type so that we generate a wrapper for it, so that
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// the C runtime can call it.
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if (!TemporaryWorkarounds && !F.isDeclaration() && F.getName() == "main") {
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Main = &F;
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LLVMContext &C = M.getContext();
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Type *MainArgTys[] = {Type::getInt32Ty(C),
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PointerType::get(Type::getInt8PtrTy(C), 0)};
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FunctionType *MainTy = FunctionType::get(Type::getInt32Ty(C), MainArgTys,
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/*isVarArg=*/false);
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if (F.getFunctionType() != MainTy) {
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LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: "
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<< *F.getFunctionType() << "\n");
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Value *Args[] = {UndefValue::get(MainArgTys[0]),
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UndefValue::get(MainArgTys[1])};
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Value *Casted =
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ConstantExpr::getBitCast(Main, PointerType::get(MainTy, 0));
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CallMain = CallInst::Create(Casted, Args, "call_main");
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Use *UseMain = &CallMain->getOperandUse(2);
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Uses.push_back(std::make_pair(UseMain, &F));
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}
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}
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}
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DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
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for (auto &UseFunc : Uses) {
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Use *U = UseFunc.first;
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Function *F = UseFunc.second;
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PointerType *PTy = cast<PointerType>(U->get()->getType());
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FunctionType *Ty = dyn_cast<FunctionType>(PTy->getElementType());
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// If the function is casted to something like i8* as a "generic pointer"
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// to be later casted to something else, we can't generate a wrapper for it.
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// Just ignore such casts for now.
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if (!Ty)
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continue;
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auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
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if (Pair.second)
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Pair.first->second = CreateWrapper(F, Ty);
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Function *Wrapper = Pair.first->second;
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if (!Wrapper)
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continue;
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if (isa<Constant>(U->get()))
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U->get()->replaceAllUsesWith(Wrapper);
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else
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U->set(Wrapper);
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}
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// If we created a wrapper for main, rename the wrapper so that it's the
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// one that gets called from startup.
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if (CallMain) {
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Main->setName("__original_main");
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Function *MainWrapper =
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cast<Function>(CallMain->getCalledValue()->stripPointerCasts());
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MainWrapper->setName("main");
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MainWrapper->setLinkage(Main->getLinkage());
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MainWrapper->setVisibility(Main->getVisibility());
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Main->setLinkage(Function::PrivateLinkage);
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Main->setVisibility(Function::DefaultVisibility);
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delete CallMain;
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}
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return true;
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}
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